1. To clone and characterize transforming DNA sequences responsible for acquisition of tumor-forming activity, DNAs from various human tumor cells were transfected onto NIH/3T3 cells. Distinct foci were induced by DNA from two out of ten lung carcinoma cell lines. Human repetitive sequences (Alu) were detected in all of the transformed cells by Southern blot hybridization. 2. A-MuLV mutants, which deleted the Bgl II 1.6 kbp fragment, lacked the ability to transform fibroblasts, indicating that this fragment is essential for fibroblast transformation. An A-MuLV mutant lacking the Bgl II 0.7 kbp fragment retained transforming activity for fibroblasts and lymphoid cells. Results of studies demonstrated that A-MuLV was capable of transforming fibroblasts in the absence of the 3' half of the A-MuLV cell-derived sequence (ab1).